I don't think I could get along with only 1 saw, but to play the game...it'd be between 3 of my saws to be the keeper.
My 372xpw purely for sentimental attachement...I've had it for years and it was my first falling saw. The power/weight ratio is good enough to be a keeper, and I can't fault the air filtration or AV.
My most likely candidate is my ms400. I've ported it, swapped the guts from a 461R oiler, and put a wrap handle on it. It usually runs a 28" lwb, but is totally capable of a 32". I contend that it's the most versatile saw in my line up, you can cut brush with it or fall trees with it.
My other possible "lifetime" saw candidate would be my 500i. I've done quite a bit of work to it, I machined the cylinder to run an 066 piston, the power is awesome on this. Weighs less than some brands 70cc offerings and will out-cut non-ported 90cc saws. The downside for me is the air filter. I'm running a Max-flow, the stock filter is atrocious...even with the Max-flow, filter cleanings are very frequent. This alone makes me prefer the "air-injected" saws like the 400, 462, Huskies, etc.